1. Field of the Invention
This invention relates to bipolar transistors, and more particularly to a self-aligned process for making bipolar transistors characterized by a low collector-base capacitance and a low extrinsic base resistance.
2. Description of the Related Art
The fabrication of bipolar transistor devices in integrated-circuit form typically involves a standard sequence of masking and alignment steps. In practice, overlay tolerances associated with these steps impose a limit on the minimum achievable size of certain critical device features.
Thus, for example, limitation inherent in such a standard device fabrication sequence dictate the minimum area of the collector-base junction of a bipolar transistor. This area determines the capacitance of the collector-base junction. In turn, this capacitance is largely determinative of the maximum speed of operation of the bipolar device.
Various self-aligned processes for making bipolar transistor devices have been developed. These do not rely on masking and alignment steps to define the collector-base junction. Hence, a device exhibiting a small collector-base junction area and low capacitance can thereby be achieved.
However, self-aligned processes as heretofore proposed for making bipolar transistors typically suffer from several disadvantages. For example, some bipolar devices made by such processes compromise the extrinsic base resistance by utilizing doped polysilicon as the base contact material. Additionally, the dopant concentration in the base link portion of the extrinsic base, by which the intrinsic base is connected to the base contact, often depends on diffusion out of the doped polysilicon base contact. As a result, the resistance of the base link is rarely optimum and is usually undesirably high. For these reasons, the extrinsic base resistance is also frequently high. Consequently, the speed of operation of even small-area collector-base junction devices made by such priorly known processes is often thereby deleteriously affected.
Accordingly, efforts have contained by workers skilled in the art aimed at trying to devise an improved process for making a bipolar transistor device. In particular, these efforts have been directed primarily at developing a self-aligned process for making a device that exhibits both a small collector-base junction area and a low extrinsic base resistance. It was realized that such efforts, if successful, would provide a small bipolar transistor having an advantageous high-speed operating characteristic.